The invention relates to a method and apparatus for calibrating a video monitor relative to an external object. More particularly, the present invention relates to a method and apparatus for calibrating any video monitor such that the size of an object displayed on the calibrated video monitor will be the same regardless of monitor size and resolution.
Video monitors generate images on a cathode ray tube (CRT) or liquid crystal display (LCD) by energizing one or more pixels on the CRT or LCD to generate light visible to the user of the monitor. A line, for example, is represented by illuminating a series of adjacent pixels. Pixels are the fundamental building blocks from which all computer graphics are created. A pixel is the smallest possible dot that can be represented on a given video monitor. All pixels on a given monitor are the same size. However, pixel size varies from monitor to monitor.
When displaying to scale the image of an object having a known size, on a given monitor, it is necessary to divide the horizontal and vertical size of the object by a scale factor that is specific to the given monitor. This scale factor is related to the pixel size.
When displaying physiological patient waveforms, e.g., electrocardiogram (ECG) or blood pressure (BP), it is standard to use a vertical scale of 10 millivolts (mV)/millimeter (mm), and a horizontal scale of 25 mm/second (sec.).
If a pixel on a given monitor is known to have a height (vertical) and width (horizontal) of 0.20833 millimeters (mm), the pixel height in a vertical direction of a 1 millivolt (mV) ECG signal, using a vertical scale of 10 mm/mV, is calculated by the following equation: EQU height of signal=(1 mV.times.10 mm/mV)/0.20833 mm/pixel EQU height of signal=48 pixels
Similarly, the pixel width in a horizontal direction (as a function of time) of a 1 mV ECG signal at a scan rate of 25 millimeters per second is calculated by the following equation: EQU width of signal/time=(25 mm/sec)/(0.20833 mm/pixel) EQU width of signal/time=120 pixels/sec